To determine if chronic suppression of pituitary prolactin will prevent or inhibit the early development of any one, more than one, or all of these types of dysplasias and as a consequence prevent or substantially reduce the incidence of mammary neoplasias. The rodents will be treated daily with the prolactin-suppressing drug 2-bromo-a-ergo-cryptine (CB-154) after the spontaneous development of, or the induction of, these mammary gland dysplasias. To determine if chronic suppression of pituitary ptolactin will prevent or inhibit the progression of any one, more than one, or all of these types of dysplasias to neoplasias and as a consequence prevent or substantially reduce the incidence of palpable mammary neoplastic lesions.